This invention relates to a multistage process for the continuous epoxidation of double bonds of terminal and non-terminal olefins containing more than 12 carbon atoms, unsaturated higher fatty acids and their esters, especially lower alkanol esters, and also unsaturated hydrocarbon alcohols containing from 8 to 18, preferably 18, carbon atoms in the alcohol radical and triglycerides of unsaturated higher fatty acids, preferably soyabean oil, with hydrogen peroxide and formic acid in the presence of a catalyst.
The epoxidation of unsaturated fatty acid derivatives, primarily soyabean oil, is carried out on an industrial scale, giving plasticizers which are compatible with polyvinyl chloride (PVC) and which also act as stabilizers, primarily against heat degradation (Ullmann's Encyclopadie der Technischen Chemie, Verlag Chemie Weinheim (1975), Vol. 10, 574). According to the Federal Republic of Germany Office of Health and the U.S. Food and Drug Administration, epoxidized soyabean oil is also accepted as an additive for plastics which come into contact with foods.
At present, the preferred epoxidizing agent is performic acid which may be formed in situ from formic acid and hydrogen peroxide. The reaction takes place in a heterogeneous mixture of peracid-containing aqueous phase and an oil phase at a slightly elevated temperature (50.degree. -80.degree. C.).
It is known that the operation of chemical processes in a continuous cascade of stirrer-equipped vessels gives poorer yields and conversions on account of the residence time spectrum than the batch process carried out in a stirrer-equipped vessel, because a corresponding conversion can only be obtained after a longer average residence time or by large excesses of one or more reactants. In the epoxidation of fatty acid derivatives, the residence time cannot be increased as required without affecting the epoxide yield, because undesirable epoxide derivatives are formed during epoxidation through ring-opening reactions which are largely initiated by the formic acid used. For the same reason, the concentration of formic acid also cannot be increased beyond a certain value.
In addition, it is necessary for reasons of industrial safety to keep the concentration of performic acid as low as possible because this strongly oxidizing acid can react explosively on heating or on contact with metals, such as for example iron or copper, or on contact with strongly reducing compounds.
The only remaining possibility of obtaining high yields in the continuous process carried out in a cascade of stirrer-equipped vessels is to increase the excess of H.sub.2 O.sub.2 by comparison with the batch process.
The quality of the olefinic starting material and of the corresponding epoxides is essentially determined by two characteristics, namely the iodine number (IN) and the epoxide value (EPO). Whereas the iodine number is a measure of the number of unsaturated double bonds, the epoxide value is an indication of the percentage content (% by weight) of epoxide oxygen. Accordingly, it is a measure of the epoxide yield. The quality of the epoxide is better, the higher the epoxide value and the lower the iodine number.
For example, a soyabean oil epoxide of very high quality is characterized by an IN of .ltoreq.2.5 and an EPO of .gtoreq.6.5, while a soyabean oil epoxide of high quality is characterized by an IN of .ltoreq.5.0 and an EPO of .gtoreq.6.3. Soyabean oil epoxide having an iodine number of more than 5.0 or an epoxide value of less than 6.3 is regarded as being of poor quality from the point of view of practical application. Accordingly the object of any epoxidation process must be to produce an epoxide which has both a low iodine number and also a high epoxide value.
Whereas a low iodine number which corresponds to a high conversion can always be obtained by a correspondingly long residence time, even with a small excess of hydrogen peroxide, there is an optimal residence time for the maximum epoxide yield. If this is exceeded, the epoxide value falls again.
In both the cascade process and also in the batch epoxidation process, the density of the heavier aqueous phase (also referred hereinafter as "acid water"), decreases as the reaction progresses while the density of the oily phase increases. The difference in density between the phases becomes very small, making the phases difficult to separate by gravity separation. In addition, the heat generated during the epoxidation reaction produces convection currents in the separator on account of temperature gradients. These convection currents are intensified by ascending gas bubbles which are formed during the decomposition of the aqueous phase.